1. Prior Art
The present invention relates generally to an organic positive temperature coefficient thermistor, and more specifically to an organic positive temperature coefficient thermistor having PTC (positive temperature coefficient of resistivity) behavior or performance that its resistance value increases drastically with increasing temperature.
2. Background Art
An organic positive temperature coefficient thermistor having PTC performance, wherein conductive particles such as carbon powders, e.g., carbon black or graphite powders, and metal powders are milled with and dispersed in a crystalline polymer, has been well known in the art, as typically disclosed in U.S. Pat. Nos. 3,243,753 and 3,351,882. The increase in the resistance value is thought as being due to the expansion of the crystalline polymer upon melting, which in turn cleaves a current-carrying path formed by the conductive fine particles.
An organic positive temperature coefficient thermistor can be used as a self control heater, an overcurrent-protecting element, and a temperature sensor. Requirements for these are that the initial resistance value is sufficiently low at room temperature (in a non-operating state), the rate of change between the initial resistance value and the resistance value in operation is sufficiently large, and the performance is kept stable even upon repetitive operations. For the organic positive temperature coefficient thermistor, it is generally known that since the melting of the crystalline polymer occurs during operation, the dispersion state of the conductive particles varies upon cooling, resulting in an increase in the initial resistance value and a decrease in the rate of resistance change.
In many cases, carbon black has been used as conductive particles in prior art organic positive temperature coefficient thermistors. A problem with carbon black is, however, that when an increased amount of carbon black is used to lower the initial resistance value, no sufficient rate of resistance change is obtainable, and when the amount of carbon black is decreased to obtain a sufficient rate of resistance change, on the contrary, the initial resistance value becomes impractically large. Sometimes, particles of generally available metals are used as conductive particles. In this case, too, it is difficult to arrive at a sensible tradeoff between the low initial resistance value and the large rate of resistance change, as is the case of carbon black.
One approach to solving this problem is disclosed in JP-A 5-47503 that teaches the use of conductive particles having spiky protuberances. More specifically, the publication alleges that polyvinylidene fluoride can be used as a crystalline polymer and spiky nickel powders can be used as conductive particles having spiky protuberances, thereby making a compromise between the low initial resistance value and the large rate of resistance change. However, the thermistor disclosed is found to have insufficient performance stability upon repetitive operations. The operating temperature achieved by use of polyvinylidene fluoride is about 160.degree. C. In applications such as secondary batteries, electric blankets, and protective elements for toilet seats and vehicle sheets, however, an operating temperature of greater than 100.degree. C. poses an immediate danger to the human body. With the safety of the human body in mind, the operating temperature must be less than 100.degree. C., and especially of the order of 60 to 70.degree. C.
U.S. Pat. No. 5,378,407, too, discloses a thermistor comprising filamentary nickel having spiky protuberances, and a polyolefin, olefinic copolymer or fluoropolymer. The publication alleges that the thermistor has low initial resistance and a large rate of resistance change, and its performance stability is well maintained even upon repetitive operations. However, the operating temperatures obtained by high-density polyethylene and polyvinylidene fluoride polymer used in the examples are about 130.degree. C. and about 160.degree. C., respectively. The publication describes that ethylene/ethyl acrylate copolymers, ethylene/vinyl acetate copolymers, ethylene/acrylic acid copolymers, etc., too, may be used. However, the publication does not disclose any example where these polymers are actually used. Although the polymers ensure an operating temperature of less than 100.degree. C., the inventors have already confirmed that the performance of the thermistor become unstable upon repetitive operations.
The thermistor disclosed in U.S. Pat. No. 4,545,926, too, uses spherical Ni, flaky Ni or rod-like Ni, and polyolefins, olefinic copolymers, halogenated vinyl or vinylidene polymers. The examples show that ethylene/ethyl acrylate copolymers and ethylene/acrylic acid copolymers ensure an operating temperature of less than 100.degree. C. while other polymers make the operating temperature greater than 100.degree. C. With the ethylene/ethyl acrylate copolymers and ethylene/acrylic acid copolymers, however, performance becomes unstable upon repetitive operations, as already mentioned.
In JP-A 10-214705, the inventors have already come up with an organic positive temperature coefficient thermistor obtained by milling polyethylene oxide having a weight-average molecular weight of at least 2,000,000 and conductive particles having spiky protuberances, thereby achieving an operating temperature of less than 100.degree. C. and making a compromise between low initial resistance and a large rate of resistance change. This thermistor is found to show excellent PTC performance and have an operating temperature of 60 to 70.degree. C. and low initial resistance in a non-operating state (room temperature), with a sharp resistance rise upon operation, a large rate of resistance change upon transition from the non-operating state to operating state, and stable performance even upon repetitive operations.
However, a problem associated with this thermistor is that its performance becomes unstable in a high-humidity environment. As will be indicated in the examples given later, some considerable degradation is found within as short as 50 hours in humidity resistance testing at 80.degree. C. and 80% RH. The reason is that the polyethylene oxide, because of being soluble in water, adsorbs water or diffuses in the polymer. However, if the thermistor is treated at high temperature to evaporate off water, then it is restored in performance. This indicates that the performance degradation is ascribable to the humidity resistance of the thermistor.